1. Field of the Invention
The present invention relates to a piston type variable displacement compressor, and more particularly, to a piston type variable displacement compressor which controls the inclined angle of a swash plate, tiltably supported on a rotary shaft, by utilizing the pressure differential between a crank chamber and a section chamber, and which converts the rotational motion of the swash plate to the reciprocal linear motion of each piston.
2. Description of the Related Art
In general, compressor units used in automobiles, trucks and the like are used to supply compressed refrigerant gas to the vehicle's air conditioning system. To maintain air temperature inside the vehicle at a level comfortable for the vehicle's passengers, it is important to utilize a compressor whose displacement amount of the refrigerant gas is controllable. One known compressor of this type controls the inclined angle of a swash plate, tiltably supported on a rotary shaft, based on the difference between the pressure in a crank chamber and the suction pressure, and converts the rotational motion of the swash plate to the reciprocal linear motion of each piston.
In the conventional compressor, an electromagnetic clutch is provided between an external driving source, such as the vehicle's engine, and the rotary shaft of the compressor. Power transmission from the driving source to the rotary shaft is controlled by the ON/OFF action of this clutch. When power transmission from the driving source to the rotary shaft is interrupted, the compressor's displacement of refrigerant gas is set to zero. At the time the electromagnetic clutch is activated or deactivated, the clutch's action generates a shock generally detrimental not only to the compressor but also to the overall driving comfort experienced by the vehicle's passengers.
To solve the above shortcoming, Japanese Unexamined Patent Publication No. 3-143725 discloses a compressor designed to set the displacement amount to zero without using an electromagnetic clutch. This compressor utilizes an increase of pressure in the crank chamber where the swash plate is accommodated, to set the inclined angle of the swash plate to zero degrees. Any quick increase in crank case pressure would, according to this design, result in a rapid and complete reduction of the compressor's gas displacement. This, in turn, would decrease the load of the compressor on the driving source. For compressor-equipped vehicles, however, it would be advantageous to use all the engine's power to drive the vehicle, especially when the vehicle is accelerating or is going uphill. In such instances, therefore, power supplied to the compressor would be temporarily interrupted to reduce the engine load produced by auxiliary machines such as the compressor.
To restore the conventional compressor's output displacement, a hydraulic actuator is used to increase the inclined angle of the swash plate. Displacement control is effected by two switches. A first electromagnetic switch, disposed in a gas passage connecting the conventional compressor's discharge and crank chambers, opens the the passage to set the displacement amount to zero. A second electromagnetic switch, disposed in an oil passage connecting an oil reserve at the bottom of the crank chamber to the hydraulic actuator, opens the oil passage to restore the displacement amount.
The installation of the two electromagnetic switches in the compressor, however, inevitably increases the size and weight of the compressor. This is a considerable disadvantage. Moreover, according to the conventional design, if the displacement mount is kept at zero, the refrigerant gas in the crank chamber tends to escape into the suction pressure area to equalize the pressure of the refrigerant gas in the compressor. Consequently, the inclination of the swash plate is subject to unpredictable and uncontrollable change making, accurate control over the compressor's displacement output difficult.